1. Field of the Invention
This invention relates to a pumping apparatus of the type used in handling thermoplastic materials such as so-called hot melt adhesives which are normally solid at room temperatures. More particularly, it is concerned with an auger-type pumping device which is capable of efficiently and safely handling thermoplastic adhesives and the like which have heretofore been impossible to handle and use on a commercial scale by virtue of their high flow and melt points. A particular feature of the invention is provision of pressure-responsive means for automatically regulating pumping efficiency by appropriate shifting of the auger, so that undue pressure buildups within the apparatus are avoided. Moreover, the pumping apparatus preferably includes means for counterbalancing the gravity shifting of the barrel and auger assembly so that material loss and equipment fouling is minimized.
2. Description of the Prior Art
For a number of years makers of automobiles and appliances have made use of high melting point thermoplastic adhesives in the construction of their products. For example, auto makers oftentimes apply strips of hot melt adhesive around the windshield frame of an automobile, whereupon the windshield is set in place and securely held therein by the adhesive. In some cases hot melt adhesives are sold in strips and are placed in position and heated. The more common practice however is to ship and handle such hot melt adhesives as a bulk product in large barrels as a solid material. The latter expedient of course necessitates the use of specialized apparatus for progressively softening the thermoplastic adhesive and pumping the same for ultimate use.
Several hot melt adhesive pumping devices have been proposed in the past for handling the bulk product. For example, one unit commonly in use provides a reciprocal, heated pressure plate in conjunction with an air-driven reciprocating pump. In the use of this device the pressure plate is lowered into contact with the bulk adhesive and the latter is removed from the container by the air pump. Separate means in the form of a fluid heat exchange apparatus is provided around the material barrel for the purpose of heating and softening the initially solid material in order to permit pumping thereof.
In practice however, a number of unresolved problems have detracted from the usefulness of such prior hot melt pumping devices. A prime deficiency in such units stems from the fact that they are normally capable of handling hot melt materials having flow points only up to about 400.degree. F. In addition, materials of very high viscosity are also difficult to efficiently pump with such prior devices. These factors represent considerable drawbacks since many modern-day adhesives having desirable sealing properties melt at considerably higher temperatures, e.g., 600.degree. - 700.degree. F., and are extremely viscous.
Another problem associated with such prior units stems from the delivery apparatus used in conjunction therewith. For instance, most of these devices employ a continuous, insulated, externally heated hose for conveying the pumped material to the ultimate site of application. These hoses are sometimes difficult and dangerous to handle and replace, and the external heating can in some cases be insufficient to keep the thermoplastic material completely flowable. In the latter case the hose can become plugged with material which generally necessitates a complete replacement of the hose.
Finally, prior pumping units have provided no means for balancing the travel thereof during pumping operations. That is, most units of this type designed for pumping of bulk hot melt adhesives include a follower plate adapted to enter an adhesive barrel vertically. A common problem is that the material, as it is being heated and rendered flowable, passes around the periphery of the follower plate by virtue of the pressure exerted on the material by the weight and action of the follower. This not only causes material losses but can severely foul the pumping equipment.
Background patents and patents specifically describing pumping units for thermoplastic material include: U.S. Pat. Nos. 3,282,469, 2,522,652, 2,630,248, 3,031,106, 196,627, 700,988, 3,113,705, 3,619,566, 2,697,190, 3,723,020, 3,764,043, 3,727,029, 1,507,516, 1,663,077, 1,977,831, 2,518,748 and 2,735,586.